Single crystal photovoltaic devices, especially silicon photovoltaic devices have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent and consume no expendable natural resources in their operation. However, the utility of such devices is limited by problems associated with the manufacture thereof. More particularly, single crystal material are: (1) difficult to produce in sizes substantially larger than several inches in diameter; (2) thicker and heavier than their thin film counterparts; and (3) expensive and time consuming to fabricate.
Recently, considerable efforts have been made to develop processes for depositing large area amorphous semiconductor films, which films can be doped to form p-type and n-type materials for the production of p-i-n type photovoltaic devices substantially equivalent to those produced by their crystalline counterparts. It is to be noted that the term "amorphous" as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions. Also, as used herein, the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, optical gap and absorption constant occur.
It is of obvious commercial importance to be able to mass produce photovoltaic devices such as solar cells. However, with crystalline cells, mass production was limited to batch processing techniques by the inherent growth requirements of the crystals. Unlike crystalline silicon, amorphous silicon and germanium alloy material can be deposited in multiple layers over large area substrates to form semiconductor devices such as solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in U.S. Pat. Nos. 4,400,409, for "A Method of Making P-Doped Silicon Films and Devices Made Therefrom"; 4,410,588, for "Continuous Amorphous Solar Cell Deposition And Isolation System And Method; 4,542,711, for "Continuous Systems For Depositing Amorphous Semiconductor Material"; 4,492,181 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells"; and 4,485,125 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells." As set forth in these patents, the disclosures of which are incorporated herein by reference, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific layer of semiconductor material. For example, in making a solar cell of n-i-p type configuration, the first chamber is dedicated for depositing a layer of n-type amorphous silicon alloy, the second chamber is dedicated for depositing a layer of intrinsic amorphous silicon alloy material, and the third chamber is dedicated for depositing a layer of p-type amorphous silicon alloy material.
The layers of thin film semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, as well as photodiodes, phototransistors, other photosensors, memory arrays, display devices or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked photovoltaic cells or other semiconductor devices of various configurations may be fabricated.
The concept of utilizing multiple stacked cells of differing band gaps to enhance photovoltaic device efficiency was created to more effectively collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell, a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap material absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, while the short circuit current thereof remains substantially constant, the overall open circuit voltage is the sum of the open circuit voltage of each cell.
Most photovoltaic device preferably also include a light reflecting back reflector for increasing the percentage of incident light reflected from the substrate back through the active semiconductor alloy material of the cells. It should be obvious that the use of a back reflector increases the amount of light which passes through the active semiconductor alloy material thus increasing the amount of incident light which is converted to electricity, and increasing the operational efficiency of the photovoltaic device. However, all layers, other than the layers of semiconductor material, deposited atop the light incident surface of the substrate must be transparent so as to pass a high percentage of incident light from the anti-reflective coating atop the photovoltaic cell to the highly reflective surface of the back reflector from which it is redirected through and absorbed by the semiconductor layers.
The back reflector may be formed atop the deposition surface of the substrate if an opaque substrate is employed. Of course, the back reflector may be either specular or diffuse. With either type of back reflector, light which has initially passed through the active body of semiconductor alloy material from which the photovoltaic device is fabricated, but which is absorbed on its initial pass, is redirected by the highly reflective material of the back reflector to pass, once again, through said active body of semiconductor alloy material. The additional pass results in increased photon absorption and charge carrier generation, thereby providing increased short circuit current. In the case of specular back reflectors, wherein the highly reflective material is conformally deposited over a smooth surface, the unused light is generally redirected for one additional pass through the active body of semiconductor alloy material of the device.
Diffuse back reflectors include a highly reflective material which is grown in a textured manner upon an underlying surface. Light incident upon said textured back reflector is scattered in addition to being redirected through the active body of semiconductor alloy material, thereby mandating that a portion of the redirected light travel at angles sufficient to cause said redirected light to be substantially confined within the photovoltaic device, i.e., achieve total internal reflection. This internal reflection provides for lengthened photon paths through the active semiconductor alloy material, thus increasing the operational efficiency of the photovoltaic device. Further, textured, diffuse back reflectors redirect light through the active semiconductor alloy material of the photovoltaic device at an angle; thus the active semiconductor alloy material appears thinner, thereby reducing charge carrier recombination, while still maintaining efficient charge carrier generation (improving short circuit current) and promoting charge carrier collection (improving the fill factor). Also, textured back reflectors contribute to textured overlying layers of semiconductor alloy material. Textured semiconductor alloy material has a larger boundary surface area, thus making the path a charge carrier must travel to collection shorter, and therefore reducing the likelihood of recombination in the active semiconductor alloy layer.
As should be apparent from the foregoing discussion, and since the purpose of a back reflector of a photoresponsive device is to redirect incident light for at least a second pass through the active body of semiconductor alloy material thereof, absorption of that incident light by the back reflector cannot be tolerated. Further, the material employed as the back reflector cannot interact with, or diffuse into, the overlying layers of semiconductor alloy material. Additionally, the back reflector material cannot add series resistance to the photovoltaic device, nor can it be too soft, since softness decreases yield, while causing short circuit junctions, and interlayer peeling. Accordingly, several different materials have been investigated for use as the back reflector is photovoltaic devices. Several are reviewed hereinbelow. For use as back reflectors, the most highly reflective material is silver which is characterized by an integrated reflectivity of about 94%. Aluminum is another highly reflective back reflector material commonly used in the fabrication of back reflectors. Aluminum has an integrated reflectivity of about 88%. Yet another highly reflective material which has been proposed for use as a back reflector is copper which is characterized by an integrated reflectivity of about 70%. The last of the most commonly employed reflective materials from which back reflectors are fabricated, is stainless steel having an integrated reflectivity of about 45%. While stainless steel is not nearly as reflective (and indeed, was not described as "highly" reflective) as aluminum, silver, and copper, it has been utilized as a substrate material and hence remains a possible candidate when economic factors are taken into consideration.
Previous attempts to employ aluminum as the highly reflective material of a back reflector for a photoresponsive device, which device included a body of amorphous silicon alloy semiconductor material have been unsuccessful because of the interdiffusion problems alluded to hereinabove. More particularly, when the amorphous silicon alloy material is deposited upon highly reflective material fabricated from aluminum, interdiffusion of the silicon and the aluminum from the contiguous layers results. Obviously the photogenerating and photoconductive properties of the body of silicon alloy material, as well as the reflective properties of the back reflector suffer.
Prior attempts at utilizing copper to fabricate the highly reflective material of the back reflector for a photoresponsive device have, likewise, proven unsatisfactory. This can be traced to the incompatibility of copper to the subsequently deposited semiconductor material fabricated from fluorinated amorphous silicon alloys. More specifically, fluorine from the body of fluorinated amorphous silicon alloy semiconductor material would react with the highly reflective copper material to form a copper:fluoride compound. As with the aluminum:silicon compound discussed in the preceding paragraph, the copper:fluoride compound deleteriously effected both the reflective properties of the copper back reflector and the photogenerative and the photoconductive properties of the semiconductor material. And as with the aluminum back reflector material described in the preceding paragraph, even when copper was sandwiched between buffer layers of titanium, or tin oxide, of a thickness specifically tailored to prevent light absorption, said buffer layers were ineffective in sufficiently preventing interdiffusion in a manner which would maintain the desired photovoltaic properties of the semiconductor material and the reflective characteristics of the back reflector.
Prior attempts to use silver as the highly reflective material from which to fabricate the back reflectors for photoresponsive devices have been unsuccessful despite the fact that the silver back reflective material presents no interdiffusion problems. However, silver and silver alloys present their own particular problem when employed as a highly reflective back reflector material, i.e., silver due to its relatively soft nature tends to easily deform, particularly during the deposition of the body of semiconductor material, thus causing a loss of any textured surface formed upon the substrate. Further, silver is expensive as compared to other back reflector materials such as for example aluminum.
Back reflectors commonly formed of the aforementioned highly reflective materials have been employed in an attempt to provide a suitable light redirecting layer for photoresponsive devices. However, as demonstrated hereinabove, each of said highly reflective materials have significant shortcomings when used as back reflector materials. It was in an attempt to alleviate such problems (primarily diffusion between elements of the highly reflective material and the body of semiconductor materials) that back reflectors formed from such highly reflective materials as aluminum, copper and silver, have been sandwiched between layers of titanium and titanium oxide, and titanium and tin oxide. However, the use of the adhesion promoting and diffusion inhibiting layers (also referred to as "buffer layers") was not totally effective. The layers had to be very thin in order to prevent the layers from absorbing incident light. However, when made very thin, these layers could not effectively prevent interdiffusion. Alternatively, if the layers were deposited so as to prevent diffusion, they would be so thick as to absorb the incident light. It was further found that in order to increase the transparency of the buffer layer, increased series resistance was also introduced into the buffer layer, thus decreasing photoefficiency of the device. Conversely, reduced series resistance increased the amount of light reflected from the surface of the back reflector.
Peeling and cracking (adhesion failure) of the body of semiconductor material from the deposition surface of the highly reflective back reflector is believed to be due to the interdiffusion of elements at the back reflector-semiconductor material interface. The elements from which the (1) highly reflective back reflector material, (2) substrate material, and (3) semiconductor material, as described supra, are fabricated, diffuse through the respective interfaces and strain, or otherwise deleteriously effect, the chemical bonds which exist between the materials from which the contiguous layers of the aforementioned interfaces are fabricated. The resultant bond strains and recombinations cause the body of semiconductor material to crack and peel off of the underlying back reflector. And even when the aforementioned adhesion promoting and diffusion limiting layers were employed to isolate the back reflector from the substrate and the body of semiconductor material, the silver, aluminum or copper material from which the highly reflective layers are fabricated, would agglomerate, thereby substantially inhibiting, if not totally preventing, the adhesion of the subsequently deposited back reflector.
In summary, the state of the back reflector art teaches both silver and aluminum back reflectors, which are either specular or diffuse. Additionally, it is known to use a buffer layer to prevent interdiffusion between the back reflector and the overlying semiconductor device. Each of these materials, and all the combinations thereof tried to date have been inadequate for various reasons. Thus, based upon the foregoing, it should be apparent that a need existed, prior to the development of the instant invention, for the development of a highly reflective back reflector adapted for use in photoresponsive devices, the material from which the back reflector was fabricated being capable of widely diffusing incident light; be relatively inexpensive and highly reflective; and avoid the problems of inherent softness, interdiffusion and added series resistance as described hereinabove.